1. Field of the Invention
This invention relates to a quaternary phosphorus compound having a salt structure between tetraphenyl borate and a phosphonium having an aromatic group attached to a phosphorus atom through an oxygen atom and useful as a curing catalyst for epoxy resin compositions. It also relates to a method for preparing the compound and a curing catalyst.
2. Prior Art
From the past, curing catalysts are blended in epoxy resin compositions comprising an epoxy resin and a phenolic resin as a curing agent. Conventional curing catalysts are imidazole derivatives, tertiary amine compounds, tertiary phosphine compounds and derivatives thereof. These curing agents, however, are less shelf stable and induce a viscosity rise when kneaded with epoxy resins, thus adversely affecting the flow of the molding composition.
JP-B 45491/1981 discloses a compound obtained by heat treating a novolak type phenolic resin and tetraphenylphosphonium tetraphenylborate (to be abbreviated as TPP-K, hereinafter) at a temperature above the softening point of the novolak type phenolic resin and continuing the heat treatment until the resin system becomes yellowish brown or brown color. By using this compound as a curing agent for an epoxy resin, there is obtained an epoxy resin composition which is shelf stable and cures into a product having improved moisture resistance. However, since this curing catalyst is less active and slow curing, it must be used in large amounts, rather detracting from the shelf stability of epoxy resin compositions.
TPP-K itself is also a useful curing catalyst which is fully potential in that reaction takes place from a certain temperature, but slow curing. Triphenylphosphine which is often used in the art is quick curing, but less shelf stable.
Conventional curing catalysts are difficult to provide an epoxy resin composition which is shelf stable, smoothly flowing, quick curing, and moisture resistant.